1. Field of the Invention
This invention relates to high vacuum water vapor cryopumps.
2. Description of the Prior Art
High vacuum chambers used for production processes, as well as research and development, are evacuated by a variety of means, including mechanical or sorption "roughing" pumps for preliminary stages, and then diffusion, turbomolecular, ion, titanium sublimation and, most recently, helium refrigeration (cryo) pumps. Those in the latter group are considered high vacuum pumps which are suitable for operation in the deeper vacuum range after the chamber has been "roughed" down to a "crossover" pressure. This "crossover" pressure depends upon the gas load tolerance of the high vacuum pump. The high vacuum pump is started long before the "crossover" pressure is reached and is usually operated continuously, but isolated from the chamber, by a large aperture valve located in a suitable manifold or port. When the high vacuum isolation valve is opened and "rough" pumping is stopped, the chamber is then pumped at a high rate by the high vacuum pump to its operating pressure level.
Gas loads in the chamber vary considerably from one application to another. Initially, the air gases including nitrogen, oxygen and water vapor are removed. Later, in the pumpdown process, water vapor becomes the dominant gas due to desorption from internal surfaces. Some plasma processes require introduction of inert gases, such as argon, and reactive gases, such as oxygen or halogens. In such cases, it is desirable to pump water vapor at high speeds without removing the introduced gases too rapidly, i.e., to selectively pump the water vapor. Water vapor can dissociate and create undesirable oxygen and hydrogen gases.
Pumps vary in their ability to remove different gas species, depending upon their operating principles. A combination of more than one type of high vacuum pump may be desireable. One solution to the problem of high water vapor loads was introduced in the 1950s by C. R. Meissner. A lightweight coil of tubing, placed directly in the vacuum chamber, is cooled by liquid nitrogen flowing therethrough. A Meissner coil cryopumps water vapor at high rates. Unfortunately, it also cryopumps CO.sub.2 when overcooled by liquid nitrogen. This disadvantage is discussed in more detail hereinbelow. The coil must by quickly warmed when the vacuum chamber is to be opened to atmosphere to preclude moisture condensation from the external environment. This is usually done using heated and pressurized nitrogen gas to expel liquid nitrogen and to warm the coil. After the chamber is unloaded, reloaded and roughed down, the Meissner coil is quickly recooled concurrent with opening the high vacuum valve and using the high vacuum pump for pumping of the chamber.
Another method of removing water vapor at high rates with limited pumping of gases, such as argon, uses a modified helium cryopump with a throttle attached to the warmer first refrigeration stage. The throttle is cold enough to cryopump water vapor, but permits argon and other "permanent" gases to pass on (at restricted flow rate) to be pumped by first and second stages.
Most high vacuum pumps are not capable of being started at atmospheric pressure but, rather, they must be isolated from the the chamber by a valve. The valve plus a manifold, (if present), and aperture between the chamber and pump all reduce gas conductance to the pump with a resulting pumping speed reduction. Larger or additional pumps can be added to handle large pumping loads. Usually, the largest load (65-95%) is water vapor. Therefore gas pumping speeds do not accurately match the load.
Meissner coils, which are placed directly in chambers and used as supplemental pumps for cryopumping water vapor, when cooled by liquid nitrogen, are costly to operate. This is due to the continuous and wasteful nitrogen consumption. Meissner coils also have some inherent safety problems and are difficult to warm up to room temperature in less than about five minutes. In addition, Meissner coils operate at such low temperatures that they cryopump or cryotrap some CO.sub.2 vapors at typical processing pressures. The chamber pressure can then seriously fluctuate if the coil temperature varies more than about 0.10 C. It is difficult to control surface temperatures this closely, and liquid entrainment in exhausting vapors occurs as a consequence.
Helium cryopumps pump water vapor about three times as fast as air, or 31/2 times as fast as argon, but still not proportional to typical gas loads. Also, because they are capture pumps, cryopumps must periodically be regenerated to dispose of captured gases. A total regeneration cycle requires about three to four hours, the chamber not being usable during this period. Care must be taken in removal of gases from the cryopump during regeneration. Cooling capacity is quite limited, making the cryopump unable to handle significant thermal radiation heat loads, e.g., viewing surfaces above about 50 degrees C. Periodic changing of a helium purifying cartridge is also required. Helium cryopumps with refrigerated throttle devices have two potential problems: (1) cryopumping action of water vapor is far removed from the vapor source and is somewhat conductance limited by an aperture and high vacuum valve, and (2) the throttle limits the pumping of all gases, including hydrogen and oxygen, which is undesirable. U.S. Pat. No. 4,535,597 describes a "Fast Cycle Water Vapor Cryopump" which minimizes the above problems of the prior art. However, there are some applications which make it difficult or impossible to use the cryopump with its two independent cooling and defrost circuits and twin thermally bonded tube construction. In some instances, it is desirable to retain an existing "Meissner" cryosurface which has a single tube passage, e.g. one designed for use with liquid nitrogen as a cooling medium, and to rapidly cool and to rapidly heat/defrost the tube using a mixed refrigerant self-refrigerating cascade system. In other instances, fabrication difficulties do not, or chamber space does not, permit the use of the twin tube design for the cryosurface. In yet other cases, stainless steel, which has a very low thermal conductivity, and hence is not suitable for the dual tube application, must be used as a construction material to preclude contamination of the high vacuum deposition chamber.
It would appear possible to have both cooling and heating fluids flow through a single tube in the "Meissner" cryosurface rather than to use a twin tube design incorporated therein. However, this creates problems. To better understand why this is not a practical design and referring to the figure, warm refrigerant fluid leaves the cryosurface 23 during the latter part of the defrost period and must return to the compressor 1 without warming the cascade heat exchangers 11 because a mixed refrigerant self-refrigerating cascade system cannot produce adequate cooling unless these exchangers 11 are at their normal cold level. Also, the discharge pressure of compressor 1 will rise unacceptably high. On the other hand, if the outlet of the cryosurface 23 is routed to bypass cascade heat exchangers 11 and to flow directly to the compressor 1, then the return refrigerant flow during normal cooling of the cryosurface 23 will not be correct for proper system operation; the refrigerator will not produce the required cooling effect.
As a next approach, it would seem obvious to provide valves to divert the refrigerant fluid leaving the cryosurface 23 either to cascade heat exchangers 11 during cooling or directly to compressor 1 during defrost. This design has problems of severely reduced reliability and increased system complexity. Small amounts of lubricating oil from the compressor 1 can reach the diverting valves at the outlet of cryosurface 23 and cause sticky valve operation and subsequent system failure. In the referenced patent the small amounts of oil which might migrate into the defrost circuit cannot cause such problems. The hot gas valve 20 remains warmer than the freezing point of oil at all times, and any residual oil is swept from the valve during each defrost cycle.